Joint replacement surgery is a long-established and well accepted mode of treatment for conditions of the human hip, including degenerative arthritis and fracture of the femoral neck. Anatomically, the hip is essentially a ball and socket joint, in which the “ball” or head of the thigh bone (femur) is inserted into and joined with a cup-shaped “socket” in the pelvic bone. Accordingly, when these bones become eroded or broken, a total hip prosthesis is typically surgically implanted to replace the damaged native bone and cartilage within the hip joint.
In essence, a complete hip prosthesis generally comprises four different structural parts, as illustrated by FIG. 1:
(i) an acetabular prosthetic implant (prosthesis), also known as an acetabular “cup” or “shell”, that replaces the native acetabulum (hip socket);
(ii) a liner that covers the inner surface of the cup, typically made of polyurethane, ceramic, or metal;
(iii) a metal stem, for insertion into the shaft of the native femur, replacing the femoral neck and providing stability and motion for the reconstructed joint; and
(iv) a metal or ceramic ball that replaces the head of the native femur.
In some embodiments parts (iii) and (iv) are provided as a single article of manufacture.
The Recurring Surgical Problem:
Successful hip prosthetic surgery requires precise intra-operative placement and positioning of replacement structures as implants within the host's native bones such that the in vivo function of the reconstructed joint is optimized biomechanically and biologically. For the surgeon, it is necessary to ensure that the replacement structural components are implanted correctly and function in situ properly in order to avoid intra-operative and post-operative complications, as well as to ensure a long-lasting action and use for the implanted prosthesis.
There are three critical parameters for achieving a successful hip arthroplasty procedure: (1) position angles of the cup; (2) position angle of the stem; and (3) longitudinal placement of the stem.
A malpositioned hip prosthesis will not adequately restore the joint's biomechanics, will not function properly, and is at increased risk of intra-operative and post-operative complications. Such complications can include, without limitation, dislocation, impingement, fracture, implant failure, aseptic loosening, subsidence, and even catastrophic outcome. A malpositioned prosthetic implant is particularly susceptible to dislocation and early loosening because the prosthesis will not be well fitted or supported within the host's native bone.
The biggest problem routinely faced by surgeons today concerning human hip replacement procedures is how to achieve proper acetabular prosthetic implant alignment. It is generally agreed among orthopedic surgeons that the ideal anatomic position (for most patients) for positioning the acetabular prosthetic implant within the native bone of the host's hip is at 45° (degrees) of inclination (see below).
A second important angle is the angle of forward flexion (see below), which ideally is at 20° (degrees) of forward flexion. More recent advanced techniques emphasize “combined anteversion” of the reconstructed hip, rather than the cup's absolute angle of forward flexion. Combined anteversion is the sum of the angle of forward flexion of the cup plus the angle of anteversion of the stem. Since there is limited space for changing the stem's angle of anteversion, adjusting the position of the cup to that of the stem is critical to improving stability of the reconstructed hip and reducing impingement.
However, precise measurement of these specific angles, and therefore proper placement of the prostheses, has been difficult to achieve, mostly because two of these angles are relative to the patient's pelvis and the patient is covered by sterile surgical drapes during the course of the hip replacement operation. It also has not been possible to monitor any change in position of the patient's pelvis that can occur after draping the patient for the surgery.
Besides the implants' angles, tension of the soft tissue surrounding the hip is another important factor in stability of the reconstructed joint. The common tendency is adding to the length or offset of the limb to make the joint stable. In many cases the soft tissue is not tight enough to provide adequate stability and allow the most suitable prosthesis with adequate length to be implanted. It is very important to be aware of amount of change in length of the leg intra-operatively to achieve a balance between leg length and offset and avoid changes in leg length more or less than what intended.
It is also critical to realize that almost all patients with arthritic and broken hips have different degrees of shortening of the leg pre-operatively, causing leg length inequality. A common expectation, at least as important as replacement of the damaged joint, is correction of this discrepancy. Following total hip arthroplasty, otherwise satisfactory clinical results can be undermined by dissatisfaction related to a change in leg length. Moreover, leg length discrepancy after total hip arthroplasty has been reported to be associated with inferior clinical outcome.
Currently there is no device to accurately measure the change in length of the leg caused by the operation. This causes another common problem with the biggest impact on patient's satisfaction, the discrepancy between length of the operative and non-operative legs.
Currently Available Surgical Options and Choices:
It is therefore somewhat surprising to recognize that conventionally available modes of avoiding malpositioned prosthetic implants remain relatively few in number. All of these currently available techniques are cumbersome, intricate and/or complex. A summary review of the presently available options is presented below.
(1) One method to facilitate insertion of an acetabular prosthetic implant is to use a visual alignment guide device that attaches to an acetabular cup impactor (a tool used to hammer a prosthetic cup into place). Once attached, the alignment guide device provides a point of reference for the operating table and the topographical surface upon which the patient rests. See FIGS. 2A and 2B.
When using this particular technique, the surgeon must assume that the patient's body lies parallel, and the pelvis lies perpendicular, to the operating table surface. The surgeon must further assume that the surface of the operating table itself is parallel to the operating room floor and the floor itself is horizontal. Based upon these assumptions, the alignment guide device is then also presumed to be parallel to the floor during the time the surgeon implants the prosthesis.
Nevertheless, the surgeon often discovers that the underlying assumptions for using the alignment guide device are false and ultimately finds that the resultant angle of inclination for the prosthetic implant is often quite different from what was expected. Thus, even when this procedure has been correctly utilized, it is not unusual for the surgeon to see a post-operative x-ray which depicts an acetabular prosthetic implant in less than an ideal aligned position, with either a markedly increased or greatly decreased angle of inclination and/or angle of forward flexion. The other underlying problem with such conventional alignment guide devices is that the measurements are only subjective, i.e., they do not have any calibrated component by which to measure the angles of inclination or forward flexion in situ.
(2) Surgeons are commonly aware that acetabular prosthetic implant malpositioning is often caused by an unrecognized/undiscovered tilting of the host's pelvis which occurs after the patient is placed in the lateral decubitus position (i.e., lying on the side of the body) for the surgery. This recognition and awareness has persuaded some surgeons to use intra-operative x-rays as a means for detecting pelvic tilt, which may occur at any time during the course of the surgical operation, and evaluating position of the acetabular implant.
However, there are multiple disadvantages in following this procedure, among which are the following: This intra-operative x-ray technique is frequently very time-consuming and can potentially increase the risk of infection owing to the introduction of non-sterile x-ray equipment into the operating theater. In addition, the obtained x-ray images (which are directed antero-posteriorly through the host's pelvis) are almost invariably of poor quality; and useful bony landmarks (such as the anterior superior iliac spine) are often obscured within the x-ray image. These obstacles and disadvantages markedly hinder the surgeon's ability to detect or accurately measure the degree of pelvic tilt.
Furthermore, the surgeon would be required to break sterile scrub in order to use a computer to detect digitally the existence and degree of pelvic tilt and implant angle. Moreover, even if a particular degree of pelvic tilt were discovered during the operation, there is no way for the surgeon to adjust the alignment of the prosthesis with the same accuracy and precision as the measurement made using computerized digital tools.
Thus, even if intra-operative x-rays were used to determine the existence and degree of pelvic tilt, this measurement would only be of limited assistance in determining the proper inclination of the acetabular implant and would not help in any meaningful way in determining the proper degree of inclination/forward flexion for the acetabular component. In effect, assessing the presence of any changes in patient pelvic tilt or cup position would require the taking and evaluation of ever more x-ray images.
(3) The intra-operative estimation of anteversion of the femoral component of a total hip arthroplasty is generally made by the surgeon's visual assessment of the stem position relative to the condylar plane of the femur. Although the generally accepted range of intended anteversion is between 10° and 20°, the surgeon's estimation of the anteversion of the femoral stem has poor precision and is often not within the intended range of 10° to 20° of anteversion. Alternatively, modular femoral components or stems with retroverted or anteverted necks could be used, but these components are much more expensive than non-modular femoral stems.
(4) Computerized navigation systems are recognized today as being useful tools for aiding with acetabular implant position. Nevertheless, the computerized navigation system itself is very expensive equipment; and it requires both a pre-operative CT (computerized tomography) scan and time-consuming pre-operative planning in order to be used effectively during the surgery.
In particular, intra-operative positioning of the digitizing frames is time-consuming and requires the placement of pins and numerous surgical incisions. Also registration with digitizing probes is very time-consuming; and this technique is always vulnerable to an unexpected software or hardware failure, which is not immediately replaceable in many instances.
Also, loosening of the implanted pins, especially in older and osteoporotic patients, is always a risk. The main risk factor is their location (inside the operating field) that leaves them at risk of getting accidentally jarred. In addition, being in the surgical field is another reason that makes computerized navigation undesirable for many surgeons since it limits the surgical field. Thus, more surgeons than ever before are still looking for an alternative and better method, especially after encountering the difficulties imposed by the computerized navigation guided system on multiple surgical occasions.
(5) Still other surgeons follow a specific and routine practice as a mode of quality control. Such surgeons consistently and invariably insert the acetabular component in the host at 35° of inclination, even though the proper goal is placement at an angle of 45°. Their rationale is simple: It is impossible to know whether or not the host's pelvis has become tilted during the surgery. Accordingly, it is anatomically better to achieve a cup position having a less-than-perfect angle of inclination, but with certainty, rather than involuntarily create too large an angle of inclination for the implanted prosthesis. In short, these surgeons knowingly preferred to err with certainty rather than with uncertainty.